The present invention relates to curable compositions, to their use, for example as casting resins in the production of air-cooled transformers and other electrical components, and to the crosslinked products obtainable by curing the compositions, which products are distinguished by the fact that they exhibit simultaneously the features of good flame retardance, high mechanical strength and low dielectric losses at high operating temperatures.
Air-cooled transformers (voltage range up to about 40 kV) are provided with a winding, the sheathing of which consists of an electrically insulating synthetic resin. In addition to providing insulation, the synthetic resin sheathing should also contribute to the mechanical strength of the windings and also have flame-retardant properties.
The critical factors for a sheathing resin for high performance transformers are the oxygen index for combustibility, the temperature at which the dielectric loss factor tan 6 is 25% at 50 Hz and the crack index value achieved, which is a measure of resistance to temperature variation.
Flame-retardant casting resins for potting air-cooled transformers are well known and are generally based on bisphenol A epoxy resins, reinforcing fillers and flame-retardants. For example, U.S. Pat. No. 3,202,947 describes flame-retardant compositions for air-cooled transformers, containing liquid bisphenol A diglycidyl ethers, hexahydrophthalic acid, hydrated alumina and tris(chloroalkyl) phosphates.
Cycloaliphatic resin systems are also known. U.S. Pat. No. 4,009,141 describes electrically insulating curable compositions consisting of selected cycloaliphatic epoxy resins and dicarboxylic anhydrides, which are reinforced with large amounts of zirconium silicate fillers and contain finely divided hydrated alumina as additional second filler. They are suitable for the encapsulating insulation of electrical components, such as, for example, of metal transformer components, or transformer bushings.
Curable, flame-retardant compositions for air-cooled transformers are also described in FR 2 630 578 B1. Those compositions contain at least 20% by weight pretreated aluminium hydroxide, based on the total composition consisting of resin, hardener and reinforcing additives. xe2x80x9cPretreatedxe2x80x9d in this context means that, by means of heat treatment, water is removed from the aluminium hydroxide in an amount of about from 0.5 to 10% by weight, based on the original weight before removal of the water.
Since in such systems the dielectric loss factor tan xcex4 increases considerably at higher temperature, those systems are not suitable for transformers having high operating temperatures.
There is therefore a need for casting resin formulations that exhibit simultaneously the features of flame retardance, low dielectric losses and good mechanical properties, especially good cracking behaviour.
That problem has now been solved by the use of cycloaliphatic systems comprising core/shell polymers, as described in EP 0 578 613 A2. It has been found that the addition of certain fillers in certain ratios and certain amounts yields potting compounds that are distinguished both by low brittleness and by a low tan xcex4 value and also by good flame retardance.
The present invention accordingly relates to curable compositions comprising
(a) a cycloaliphatic epoxy resin that is liquid at RT and, suspended therein, a core/shell polymer,
(b) a polycarboxylic anhydride and
(c) fillers,
wherein the composition is flame-retardant because two different fillers (c1) and (c2) are present, the nature of filler (C1) being such that, starting at RT, it is able to release water as the temperature rises, the total proportion of fillers (C1) and (c2) is from 58 to 73% by weight, based on the total amount of components (a), (b), (C1) and (c2), and the ratio by weight of the fillers (C1):(c2) is from 1:3 to 1:1.
The compositions according to the invention are resin systems of moderate to relatively high viscosity that can be fully cured by heat. In the cured state they are thermosetting materials of relatively high rigidity having a glass transition temperature of about from 80 to 140xc2x0 C. The term xe2x80x9ccycloaliphatic epoxy resinxe2x80x9d in the context of this invention denotes any epoxy resin having cycloaliphatic structural units, that is to say it includes both cycloaliphatic glycidyl compounds and xcex2-methylglycidyl compounds-as well as epoxy resins based on cycloalkylene oxides. xe2x80x9cLiquid at room temperature (RT)xe2x80x9d is to be understood as meaning pourable compounds that are liquid at 25xc2x0 C., i.e. are of low to medium viscosity (viscosity less than about 20 000 mPaxc2x7s).
Suitable cycloaliphatic glycidyl compounds and xcex2-methylglycidyl compounds are the glycidyl esters and xcex2-methylglycidyl esters of cycloaliphatic polycarboxylic acids, such as tetrahydrophthalic acid, 4-methyltetrahydrophthalic acid, hexahydrophthalic acid, 3-methylhexahydrophthalic acid and 4-methylhexahydrophthalic acid.
Further suitable cycloaliphatic epoxy resins are the diglycidyl ethers and xcex2-methylglycidyl ethers of cycloaliphatic alcohols, such as 1,2-dihydroxycyclohexane, 1,3-dihydroxycyclohexane and 1,4-dihydroxycyclohexane, 1,4-cyclohexanedimethanol, 1,1-bis(hydroxymethyl)cyclohex-3-ene, bis(4-hydroxycyclohexyl)methane, 2,2-bis(4-hydroxycyclohexyl)propane and bis(4-hydroxycyclohexyl)sulfone.
Examples of epoxy resins having cycloalkylene oxide structures are bis(2,3-epoxycyclopentyl) ether, 2,3-epoxycyclopentylglycidyl ether, 1,2-bis(2,3-epoxycyclopentyl)ethane, vinyl cyclohexene dioxide, 3,4-epoxycyclohexylmethyl 3xe2x80x2,4xe2x80x2-epoxycyclohexanecarboxylate, 3,4-epoxy-6-methylcyclohexylmethyl 3xe2x80x2,4xe2x80x2-epoxy-6xe2x80x2-methylcyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl) adipate and bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate.
Preferred cycloaliphatic epoxy resins are bis(4-hydroxycyclohexyl)methanediglycidyl ether, 2,2-bis(4-hydroxycyclohexyl)propanediglycidyl ether, tetrahydrophthalic acid diglycidyl ester, 4-methyltetrahydrophthalic acid diglycidyl ester, 4-methylhexahydrophthalic acid diglycidyl ester, 3,4-epoxycyclohexylmethyl 3xe2x80x2,4xe2x80x2-epoxycyclohexanecarboxylate and especially hexahydrophthalic acid diglycidyl ester.
The cycloaliphatic epoxy resins can also be used in combination with aliphatic epoxy resins. As xe2x80x9caliphatic epoxy resinsxe2x80x9d it is possible to use epoxidation products of unsaturated fatty acid esters. It is preferable to use epoxy-containing compounds derived from mono- and poly-fatty acids having from 12 to 22 carbon atoms and an iodine number of from 30 to 400, for example lauroleic acid, myristoleic acid, palmitoleic acid, oleic acid, gadoleic acid, erucic acid, ricinoleic acid, linoleic acid, linolenic acid, elaidic acid, licanic acid, arachidonic acid and clupanodonic acid.
For example, there are suitable the epoxidation products of soybean oil, linseed oil, perilla oil, tung oil, oiticica oil, safflower oil, poppyseed oil, hemp oil, cottonseed oil, sunflower oil, rapeseed oil, polyunsaturated triglycerides, triglycerides from euphorbia plants, groundnut oil, olive oil, olive kernel oil, almond oil, kapok oil, hazelnut oil, apricot kernel oil, beechnut oil, lupin oil, maize oil, sesame oil, grapeseed oil, lallemantia oil, castor oil, herring oil, sardine oil, menhaden oil, whale oil, tall oil and derivatives thereof.
Also suitable are higher unsaturated derivatives that can be obtained by subsequent dehydrogenation reactions of those oils.
The olefinic double bonds of the unsaturated fatty acid radicals of the above-mentioned compounds can be epoxidised in accordance with known methods, for example by reaction with hydrogen peroxide, optionally in the presence of a catalyst, an alkyl hydroperoxide or a peracid, for example performic acid or peracetic acid. Within the scope of the invention, both the fully epoxidised oils and the partially epoxidised derivatives that still contain free double bonds can be used for component (a).
Preference is given to the use of epoxidised soybean oil and epoxidised linseed oil.
When cycloaliphatic epoxy resins are used in combination with aliphatic epoxy resins, the advantageous ratio by weight of cycloaliphatic to aliphatic component is from 1:0 to 0.6:0.4.
The cycloaliphatic epoxy resins used according to the invention comprise so-called core/shell polymers in suspended form as tougheners, the tougheners being liquid or solid in the starting state. They should not contain reactive groups that could react with the epoxy groups of the epoxy resin in question. It is preferable to use solid tougheners. They have the advantage that the particle size and also the proportion of toughening phase in the suspension are preset, whereas in the case of liquid tougheners the required second phase is formed during the curing with the epoxy resin.
Core/shell polymers generally have a soft core of an elastomeric material that is insoluble in the epoxy resin. Grafted onto that core is a shell of polymeric material that does not contain any groups capable of reacting with the epoxy resin.
Examples of elastomers that can be used as core material are polybutadiene, polyacrylic acid esters and polymethacrylic acid esters and co- or ter-polymers thereof with polystyrene, polyacrylonitrile or polysulfide.
Examples of polymeric shell materials are polystyrene, polyacrylonitrile, polyacrylate and methacrylate homo-, di- or ter-polymers and styrene/acrylonitrile/glycidyl methacrylate terpolymers.
Preference is given to suspensions comprising a solid core/shell polymer.
The size of such core/shell particles is advantageously from 0.05 to 30 xcexcm, preferably from 0.05 to 15 xcexcm. Core/shell particles less than 1 xcexcm in size are especially used.
The core/shell polymers can be produced, for example, in the manner described in U.S. Pat. No. 4,419,496 or EP-A 0 045 357.
Especially preferred is the use of core/shell polymers that contain a core of polybutadiene or polybutadiene/polystyrene. Such a core material is preferably only partially crosslinked. Further core materials are polyacrylates and polymethacrylates, especially polyacrylic acid esters and polymethacrylic acid esters and di- or ter-polymers thereof.
The shell consists especially of polymers based on methyl methacrylate, cyclohexyl methacrylate, butyl acrylate, styrene and methacrylonitrile, but especially based on polymethyl methacrylate.
The amount of toughener in the suspensions according to the invention that comprise a cycloaliphatic or aliphatic epoxy resin is preferably from 1 to 30% by weight, especially from 5 to 10% by weight, based on the epoxy resin.
For curing the compositions according to the invention, polycarboxylic anhydrides are used.
They may be linear aliphatic polymeric anhydrides, for example polysebacic polyanhydride or polyazelaic polyanhydride, or cyclic carboxylic anhydrides.
Cyclic carboxylic anhydrides are especially preferred.
Examples of cyclic carboxylic anhydrides are:
succinic anhydride, citraconic anhydride, itaconic anhydride, alkenyl-substituted succinic anhydrides, dodecenylsuccinic anhydride, maleic anhydride and tricarballylic anhydride, a maleic anhydride adduct with cyclopentadiene or methylcyclopentadiene, a linoleic acid adduct with maleic anhydride, alkylated endoalkylenetetrahydrophthalic anhydrides, methyltetrahydrophthalic anhydride and tetrahydrophthalic anhydride, the isomeric mixtures of the two latter compounds being especially suitable. Especially preferred are hexahydrophthalic anhydride and methylhexahydrophthalic anhydride.
The compositions according to the invention can optionally additionally comprise a curing accelerator. Suitable accelerators are known to the person skilled in the art. Examples that may be mentioned are:
complexes of amines, especially tertiary amines, with boron trichloride or boron trifluoride;
tertiary amines, such as benzyldimethylamine;
urea derivatives, such as N-4-chlorophenyl-Nxe2x80x2,Nxe2x80x2-dimethylurea (monuron);
unsubstituted or substituted imidazoles, such as imidazole or 2-phenylimidazole.
Preferred accelerators are tertiary amines, especially benzyldimethylamine, and imidazoles (e.g. 1-methylimidazole) for the above-mentioned compositions that comprise epoxidised oils.
The curing agents and, where applicable, accelerators are used in the customary effective amounts, that is to say amounts sufficient for curing the compositions according to the invention. The ratio of the components resin system/hardener/accelerator depends upon the nature of the compounds used, the rate of curing required and the properties desired in the end product and can readily be determined by the person skilled in the art. Generally, from 0.4 to 1.6 equivalents, preferably from 0.8 to 1.2 equivalents, of anhydride groups per epoxy equivalent are used.
The curing accelerators are usually used in amounts of from 0.1 to 20 parts by weight per 100 parts by weight of epoxy resin.
As component c1 the compositions according to the invention comprise fillers having flame-retardant properties. Such fillers exhibit flame-retardant properties because their nature is such that, starting at room temperature, they are able to release water as the temperature rises. There are therefore suitable, for example, aluminium hydroxide, water-containing magnesia or zinc borate or other substances that decompose with the release of water at elevated temperatures.
It is preferable to use aluminium hydroxide, which may be either untreated or thermally pretreated and/or silanised Al(OH)3. xe2x80x9cThermally pretreatedxe2x80x9d in this context means that, by means of heat treatment, water is removed from the aluminium hydroxide advantageously in an amount of from about 0.5 to about 10% by weight, based on the original weight before removal of the water. Methods in this connection are described in FR 2 630 578 B1.
In order to obtain the desired mechanical strength, the starting resin is actively reinforced by the addition of a further filler c2 which is different from c1. Examples of suitable reinforcing materials c2 are glass fibres or carbon fibres. The following materials, for example, also come into consideration as component c2: metal powder, wood flour, glass powder, glass beads, semi-metal and metal oxides, such as SiO2 (quartz sand, quartz powder, silanised quartz powder, fused silica powder, silanised fused silica powder), aluminium oxide, titanium oxide and zirconium oxide, semi-metal and metal nitrides, for example silicon nitride, boron nitrides and aluminium nitride, semi-metal and metal carbides (SiC and boron carbides), metal carbonates (dolomite, chalk, CaCO3), metal sulfates (barytes, gypsum), ground minerals, and natural or synthetic minerals chiefly of the silicate series, e.g. zeolites (especially molecular sieves), talcum, mica, kaolin, wollastonite, silanised wollastonite and others.
Preferred fillers c2 are quartz powder, silanised quartz powder, wollastonite and silanised wollastonite, both on their own and in combination.
Wollastonite is a naturally occurring acicular calcium silicate of the formula Ca3[Si3O9] having particle sizes in the micron range. Artificially produced wollastonite is also acicular. Wollastonite is commercially available, for example under the name Nyad(copyright) from the Nyco company.
The total proportion of components (c1) and (c2) in % by weight is from 58 to 73% by weight, preferably from 63 to 68% by weight, based on the total amount of components (a), (b), (C1) and (c2), and the ratio by weight of the fillers (C1):(c2) is from 1:3 to 1:1, preferably from 1:2.3 to 1:2.
If desired, in addition to fillers c1 and c2 it is also possible to use a wetting and dispersing agent, which reduces the mainly electrostatic interactive forces between resin and filler and the increased viscosity caused thereby.
The wetting and dispersing agent is advantageously used in an amount of about from 0.1 to 2.0% by weight, based on the total amount of components (a) and (b).
In addition to the fillers c1 and c2 mentioned above and, where applicable, a wetting and dispersing agent, the curable mixtures may comprise further customary additives, e.g. antioxidants, light stabilisers, fillers containing water of crystallisation, plasticisers, dyes, pigments, fungicides, thixotropic agents, antifoams, antistatics, lubricants, anti-settling agents, wetting agents and mould-release aids.
The compositions according to the invention can be produced in accordance with known methods using known mixing apparatus, for example stirrers, kneaders and rollers. The curing of the mixtures according to the invention can be carried out in known manner in one or more steps. It is generally effected by heating to temperatures of from 60xc2x0 C. to 200xc2x0 C., especially from 80xc2x0 C. to 180xc2x0 C. When curing is carried out in two or more steps, it means that curing is effected in stages, each at a higher temperature.
The invention accordingly relates also to crosslinked products obtainable by curing a composition according to the invention.